Radar and other active range determination systems are in widespread use for military, commercial, and private purposes. Radar systems have well-known characteristics, in that long-range detection of small targets is known to require transmission of more power, higher-gain antennas, and or more sensitive receivers than that or those required for short-range detection of large targets. Among the characteristics of radar systems used for detecting targets at long range are those relating to range ambiguity, which has to do with reception of signals returned from a target lying beyond the range defined by the pulse repetition interval, which may make the distant target appear to be near the radar system. Another such characteristic of radar is that of range eclipsing, which has to do with the inability of a radar receiver to receive return signals during the pulse transmission interval.
A conventional solution to range eclipsing is to vary the pulse repetition interval, so that the transmitted pulses are staggered over time, thereby allowing the receiver to periodically “see” returned signals at times which would otherwise be lost or eclipsed. The eclipsing still occurs for each individual pulse train, but the totality of the radar returns over time includes information which fills in the gaps attributable to the individual transmitted pulse trains. The tradeoff is that a longer time is required to produce all the information required for an uneclipsed view of the region. Another possible solution to range eclipsing is to reduce the duty cycle of the radar by reducing the transmitted pulse duration, to thereby reduce the duration of the eclipsing. The reduction of the pulse duration, however, tends to reduce the transmitted energy, which reduces the range sensitivity, which again requires a longer period of integration in order to obtain the same effective range.
Another possible solution to range eclipsing is to reduce the duty cycle of the radar by increasing the pulse repetition interval, to thereby move the increased range interval to a distant range not of interest. The reduction of the duty cycle and increase in the pulse repetition interval, however, tends to consume additional radar resources resulting in a greater overall time required for completion of a surveillance scan.
Conventional range ambiguity resolution techniques require transmission of additional signals with additional dwells for resolving the range interval of the ambiguous target. The additional dwells or transmissions consume additional radar resources, resulting in a greater overall time required for completion of a surveillance scan. A radar system's maximum unambiguous range (Rmax) is given by
                              R          max                =                  C          *                      PRI            2                                              (        1        )            where:
C is the speed of light;
* represents simple multiplication; and
PRI is the radar pulse repetition interval in seconds.
This is the maximum range from which a reflection can be received from a target before the next pulse is transmitted. It is possible for reflections from objects lying at distances greater than Rmax to arrive at the radar receiving antenna and receiver at a time after the transmission of a later pulse. The signals received from such a remote target tend to be weak, and may be obscured by reflections from a closer target or clutter.
U.S. Pat. No. 6,639,546, issued Oct. 28, 2003 in the name of Ott et al. describes sequential transmission of pulses at different frequencies, together with selective reception at the different frequencies and processing of the returns for reducing eclipsing and range ambiguity.
Improved or alternative active ranging systems are desired.